Aromatic polycarbonates having relatively high modulus, flexural strength and secondary transition temperatures are well known in the art. One of the methods known of raising the secondary transition temperature of polycarbonates while substantially maintaining a significant number of the other properties of polycarbonate is to incorporate a significant number of ester bonds, particularly aromatic ester bonds. In this manner, a copolyestercarbonate is produced, see Goldberg U.S. Pat. No. 3,169,121 incorporated by reference. Copolyestercarbonate with relatively high secondary transition temperatures are known, see Miller U.S. Pat. No. 4,465,820 and U.S. Pat. No. 4,464,512, for example. However, none of these disclosures appear to mention the effect of having a substituent on the aromatic ester ring other than the usual "hydrocarbyl", "chloro", "alkyl", etc. groups.
A substantially sized, double heteroatom group has now been placed into the backbone of a copolyestercarbonate, the substantially sized double heteroatom group functioning as a side group instead of joining one repeating unit to another. This group provides a high flexural modulus as well as a high secondary transition temperature with fewer ester units than necessary in prior copolyestercarbonates.